1. Field of the Invention
The present invention relates to a changeable pipette tip, a pipetting device, a pipette tip actuating device and a method for pipetting which allow to take in liquid and to eject liquid volumes in the nanoliter range.
2. Description of the Related Art
According to prior art, pipetting methods with changeable tips only allow to dose volumes in the range of some microliters (10−9 m3) to milliliters (10−6 m3).
A typical hand pipette 10 with a changeable pipette tip 12 is illustrated in FIG. 5. The hand pipette 10 includes a coupling means 14 onto which the rear part of the pipette tip 12 may be plugged so that there is a fluid connection of the internal fluid areas of the pipette tip with internal fluid areas of the hand pipette 10 via the fluid opening 16. The hand pipette 10 includes means (not shown) for generating a negative pressure and/or a positive pressure in its internal fluid areas so that liquid may be sucked into the pipette tip and/or ejected therefrom through an orifice opening 18 in the pipette tip 12.
Automatic pipetting devices generally provide a moveable mount 20 (FIG. 6) comprising coupling means 22 for receiving one or more pipette tips 24. Here, the pipette tips 24 are connected to the pipetting means comprising the moveable mount 20 such that liquid is sucked in and/or ejected through the orifice openings of the pipette tips 24 by corresponding actuating means in the pipetting means. For this purpose, fluid areas in the pipette tip are generally in fluid connection with fluid areas in the pipetting means.
In such automatic pipetting devices, the pipette tip 24 is taken from a carrier and clamped into the moveable mount 20. For aspirating, the orifice opening of the pipette tip is then immersed in a vessel. After the liquid has been taken in, which is achieved by means of negative pressure in the pipette, the pipette tip is moved over the target where then, by means of a positive pressure in the pipette, either its whole contents or a small part thereof is released into the target vessel. In the case of large volumes, this may be done in a free jet, in the case of small volumes there may have to be a contact between target and pipette tip since there will be no drop break due to adhesion forces at the pipette tip.
On the one hand, the above process limits the minimum volume to be released, on the other hand, there may also occur a carry-over of substances already present in the target. Once the dosing is completed, the pipette tip is released into a waste box by means of an automatic ejection mechanism. In the example shown in FIG. 6, the automatic pipetting device, as it is used by the company Eppendorf, for example, includes coupling means for eight pipette tips.
For the release of liquid volumes of less than one microliter, the prior art knows free jet methods, as described in DE 19802367 C1 and DE 19802368 C1. There, a pressure chamber is respectively bounded by a membrane on one side, so that liquid droplets may be ejected from an ejection opening fluidically connected to the pressure chamber by deflecting the membrane.
A method allowing to release liquid amounts in the range of some 10 nanoliters is known as so-called “mosquito” method by the company TP Labtech, wherein here, however, the part to be exchanged in fluid contact involves a lot of manufacturing effort and is correspondingly expensive. Other systems, such as one described in EP 1093856 A1, are based on adapted pipettes into which a membrane is inserted. By means of an actuator pushing it, it allows higher dynamics than air cushion pipettes, whereby smaller release amounts may be achieved.
Finally, a method for dosing liquids in the nanoliter range is known from the German application 10337484.1, which was not prepublished. Such a method is schematically shown in FIGS. 7a to 7c. As shown in FIG. 7a, a flexible tube 30 includes an end 32 on the inlet side serving for the connection to a liquid reservoir, and an end 34 on the outlet side at which micro drops and/or micro jets may be released. Respective walls 36 of the elastic tube 30 are illustrated by dashed lines. An actuating means 38 is provided in the form of a displacer, which comprises a connecting part 40 with which the displacer 38 may be attached to an actuating member for driving the displacer 38.
The elastic tube may, for example, have a substantially constant cross-section, which will generally be circular, from its inlet end 32 to its outlet end 34. An area 42 arranged below the displacer 38 may be referred to as dosing chamber area and is defined by the position of the displacer with respect to the elastic tube. An area 44 essentially starting at the right end of the displacer 38 represents an outlet channel, while an area 46 essentially starting at the left end of the displacer 38 represents an inlet channel. The displacer 38 may comprise a displacer surface 50 extending in an inclined way with respect to the wall 36 of the flexible tube 30, which, during operation of the micro dosing device, allows the generation of a preferred direction in the direction of the outlet opening 34.
Assume that, in the state shown in FIG. 7a, the flexible tube 30 is filled with a liquid, wherein such a filling may be done, for example, by capillary forces. Starting from this state, the displacer 38 is moved quickly downwards in the direction of the arrows 52, so that a reduction of the duct volume between inlet opening and outlet opening is caused. This results in a liquid flow 54 towards the outlet end 34 and a liquid flow 56 towards the inlet end 32. Due to the forward flow 54, there is a liquid ejection in the form of a micro drop 60 and/or micro jet at the outlet opening 34. The portion of the liquid released through the outlet opening 34 as jet and/or drop depends on the position, type and dynamics of the volume change. Furthermore, the amount of liquid released as drop depends on the size of the displacer and on the stroke of the displacer 38, i.e. how much the tube is compressed.
After the ejection process, there is a refilling phase in which the displacer 38 is moved away from the tube in the direction of arrow 61 so that the volume of the inlet opening 32 and the outlet opening 34 is increased again and thus liquid flows through the inlet channel 46, see arrow 64 in FIG. 7c. 
The details regarding this dosing method are described, as mentioned above, in the German application 10337484, whose related disclosure is incorporated by reference.
U.S. Pat. No. 5,032,343 discusses a micro pipette tip for locations that are hard to reach. For this purpose, the front portion of the micro pipette tip is extended and ultra thin. This causes flexibility allowing the placement of the pipette tip at locations that are hard to reach. The opposing end of the pipette tip is rigid and allows to attach the pipette tip to conventional tools.
U.S. Pat. No. 6,180,061 discusses a cartridge pump and dispensing arrangement for applications in which cartridges containing liquid reagents are changed frequently. The cartridge pump includes a reservoir in which there is arranged a moveable piston. A lower open end of the reservoir empties directly into a dosing chamber in the form of a flexible pipe line. An actuating means in the form of a rubber mallet is provided by which the volume of the dosing chamber may be reduced to thereby cause an outflow of liquid through a nozzle connected to an end of the dosing chamber on the ejection side. Two valves are further arranged in the dosing chamber which allow only a unidirectional flow from the end on the reservoir side to the end on the nozzle side of the dosing chamber.
WO 02/092228 A2 discusses a dispensing means in which a conventional syringe pump is used to eject liquid volumes of less than 5 μl. According to this document, there is provided a syringe pump that is connected to a dispenser 2 via a tube line. The dispenser includes a system liquid reservoir separated from a sample liquid reservoir by an elastomer membrane. A nozzle comprising a nozzle bore is connected to the sample liquid reservoir. The system liquid reservoir is connected to the syringe pump via the tube line so that, by actuating the syringe pump, the membrane may be deflected via the system liquid to thereby eject sample liquid through the nozzle. Sample liquid may also be sucked in through the nozzle, wherein, in order to disconnect a droplet hanging at the nozzle after sucking in the sample liquid, an electric field is applied between the nozzle and a suitably arranged counterelectrode. One embodiment alternatively provides an actuating means to cause an axial expansion and/or compression of the nozzle to thereby disconnect droplets adhering to the tip after the sucking.
EP 0028478 B1 discusses a pipetting means in which an elastic pipe line is housed in a block such that a pipe-shaped recess in the block has a larger diameter than the outer diameter of the elastic pipe line. An end of the elastic pipe line on the ejection side is connected to a connection piece which, in turn, is connected to a pipette tip. Sucking in and ejecting liquid through the pipette tip is done by applying a positive pressure or a negative pressure to the internal bore of the block to cause corresponding volume changes of the elastic pipe line. In one embodiment, an end of the described squeeze tube means spaced apart from the pipette tip is connectable to a syringe via a three-way valve so that sample liquid and a diluting agent that was drawn into the syringe beforehand may be ejected into an outflow vessel through the pipette tip by actuating the syringe and by pressurizing the internal bore of the block.